Acoustic decoupling in cMUTs

1. A capacitive micromachined ultrasonic transducer (cMUT), comprising:

a base having a lower base portion and an upper base portion, the lower base portion and the upper base portion defining a cavity therebetween, the upper base portion having a first electrode; and

a movable layer disposed above the upper base portion and supported by a support connected to the upper base portion, the support spacing the movable layer from the upper base portion to define a transducing space, the movable layer having a second electrode opposing the first electrode to evacuate a variable capacitor between the base and the movable layer.

2. The cMUT as recited in claim 1 , wherein the lower base portion comprises a substrate.

3. The cMUT as recited in claim 1 , wherein the cavity is formed on the lower base portion by removing a part of the lower base portion.

4. The cMUT as recited in claim 1 , wherein the upper base portion comprises a separate layer placed over the cavity.

5. The cMUT as recited in claim 1 , wherein the bottom electrode comprises a separate layer placed over the cavity, the separate layer being electrically conductive.

6. The cMUT as recited in claim 1 , wherein the upper base portion comprises a support layer placed over the cavity, the support layer being electrically non-conductive, and wherein the first electrode comprises a conductive layer placed on the support layer.

7. The cMUT as recited in claim 1 , wherein the movable layer comprises a membrane, and wherein the second electrode is placed on the membrane.

8. The cMUT as recited in claim 1 , wherein the upper base portion has a thickness that is small enough to result in an acoustic ring frequency that is substantially separated from an operating frequency of the cMUT.

9. The cMUT as recited in claim 1 , wherein the cMUT is divided by trenches into a plurality of cMUT elements each having a variable capacitor element and a cavity element below the variable capacitor element.

10. The cMUT as recited in claim 9 , wherein the cMUT includes a surface plate and a spring layer, and the trenches are formed through the surface plate, the spring layer and the upper base portion.

11. A capacitive micromachined ultrasonic transducer (cMUT), comprising:

a base having a lower base portion and an upper base portion, the lower base portion and the upper base portion defining a cavity therebetween;

a spring layer disposed above the upper base portion and anchored by a spring anchor connected to the upper base portion, the spring anchor spacing the spring layer from the upper base portion to form a cantilever-like structure having an anchored portion and a movable portion; and

a surface plate disposed above the spring layer and supported by a spring-plate connector connected to the movable portion of the spring layer, the spring-plate connector spacing the surface plate from the spring layer to define a transducing space, wherein the surface plate includes a top electrode, and wherein at least one of the spring layer, the spring anchor and the upper base portion includes a bottom electrode opposing the top electrode to evacuate a variable capacitor therebetween.

12. The cMUT as recited in claim 11 , wherein the lower base portion comprises a substrate.

13. The cMUT as recited in claim 11 , wherein the cavity is formed on the lower base portion by removing a part of the lower base portion.

14. The cMUT as recited in claim 11 , wherein the upper base portion comprises a separate layer placed over the cavity.

15. The cMUT as recited in claim 11 , wherein the bottom electrode comprises a separate conductive layer which is a part of the spring layer.

16. The cMUT as recited in claim 11 , wherein the bottom electrode is an integral part of the spring layer.

17. The cMUT as recited in claim 11 , wherein the cMUT is divided by trenches into a plurality of cMUT elements each having a variable capacitor element and a cavity element below the variable capacitor element.

18. The cMUT as recited in claim 17 , wherein the trenches are formed through both the surface plate and the spring layer.

19. The cMUT as recited in claim 17 , wherein the trenches are formed through the surface plate, the spring layer and the upper base portion.

20. A method for fabricating a capacitive micromachined ultrasonic transducer, the method comprising:

providing a substrate;

forming a cavity on a surface of the substrate;

placing a cover layer over the cavity; and

completing a cMUT structure on top of the cover layer, wherein the cMUT structure includes a spring layer disposed above the cover layer and a first electrode and a second electrode defining a variable capacitor, at least one of the first electrode and the second electrode being movable to vary the variable capacitor.

21. The method as recited in claim 20 , wherein the first electrode resides in or on the cover layer.

22. A method for fabricating a capacitive micromachined ultrasonic transducer, the method comprising:

providing a substrate;

forming a cavity on a surface of the substrate;

placing a cover layer over the cavity;

forming a spring anchor on the cover layer;

placing a spring layer over the spring anchor, when placed the spring layer having an anchored portion and a movable portion, the anchored portion being anchored and the spring anchor;

forming a spring-plate connector on the movable portion of the spring layer; and

placing a surface plate over the spin-plate connector,

wherein the cMUT structure includes a first electrode and a second electrode defining a variable capacitor, at least one of the first electrode and the second electrode being movable to vary the variable capacitor.